1. Field of the Invention
Exemplary aspects of the present disclosure generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photosensitive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
There is known a tandem-type color image forming apparatus in which a plurality of photosensitive members are arranged in tandem facing an intermediate transfer belt of a transfer unit entrained around and stretched taut between a plurality of rollers. In such an image forming apparatus, toner images of different colors are formed on each of the photosensitive members and transferred onto the intermediate transfer belt such that they are superimposed one atop the other, forming a composite toner image in a process known as a primary transfer process. Subsequently, the composite toner image is transferred onto a recording medium such as paper in a process known as a secondary transfer process.
Such a known tandem-type image forming apparatus has multiple printing modes, for example, a single color or monochrome mode using one photosensitive member for forming a single color image and a multiple color mode using a plurality of photosensitive members for forming a color image, and these modes are variable. In this configuration, during the single color mode, the photosensitive members that are not used are separated from the intermediate transfer belt to prevent deterioration of the photosensitive drums and the intermediate transfer belt.
In one example of a known separation technique employed in the tandem-type image forming apparatus, the intermediate transfer belt is stretched taut linearly between two belt support rollers, one of which disposed at the photosensitive member side not to be used is held movably by a holder. A rod of a solenoid contacts the holder. The rod projecting from the solenoid contacts the holder to position the belt support roller at its contact position at which the intermediate transfer belt contacts the photosensitive drum. In a case in which the rod contacts the holder while the rod does not project from the solenoid, the roller is at its separating position at which the intermediate transfer belt is separated from the photosensitive drum.
In another approach, primary transfer rollers are disposed each facing the photosensitive members via the intermediate transfer belt, and the primary transfer rollers facing the photosensitive members not to be used are movably held by a holder, and the holder contacts a cam. As the position of the cam changes, the primary transfer rollers take either the contact position or the separating position.
According to the known approaches described above, generally, a frame member holds the belt support rollers and the primary transfer rollers, and is disposed both at a proximal side and a distal side in the axial direction of the rollers. Furthermore, the transfer unit is positioned in place in the image forming apparatus by using a reference member provided at a reference position of the frame member. The frame member rotatably holds the belt support rollers and the primary transfer rollers at a position a certain distance from the reference position.
However, in reality, the frame member of the transfer unit itself has a tolerance, and the rollers such as the belt support rollers and the primary transfer rollers have a mounting tolerance or the like relative to the frame member so that the positional accuracy of the rollers depends on these tolerances.
The solenoid and the cam described above are mounted at a position a certain distance from the reference position of the frame member. As a result, the solenoid and the cam also have a mounting tolerance. In such a configuration, parallelism between the roller to be separated and the roller not to be separated is difficult to achieve because the mounting tolerance of the solenoid and the cam are added to the tolerance of the frame member itself and the mounting tolerance of the roller to be separated and the roller not to be separated. Consequently, the intermediate transfer belt moves improperly or shifts easily.
Of course, when using a solenoid and a cam, parts required for mounting the solenoid and the cam are required, hence increasing the number of parts and complicating efforts to achieve desirable positional accuracy of the rollers to be moved. As a result, the tension of the intermediate transfer belt is difficult to stabilize, and the moving speed of the intermediate transfer belt fluctuates easily. Fluctuation of the moving speed of the intermediate transfer belt results in unevenness of image density, and color drift occurs in an output image in the multiple color mode.
In view of the above, there is demand for an image forming apparatus that can prevent improper movement and fluctuation of the intermediate transfer belt when moving the intermediate transfer belt in multiple color mode and the single color mode.